The present invention relates generally to a an analysis system for use with an electrical utility power detection system for detecting high impedance, low current arcing faults on the power system. Power system faults may be caused by, for example, downed, broken, tangled or dangling power lines, trees contacting the power lines, and various overcurrent fault situations.
High impedance, low current arcing faults are more difficult to detect than permanent overcurrent faults, which for instance, occur when a transformer fails. Most conventional overcurrent protection devices, such as fuses, reclosers, relays and the like, have time delays which prevent a temporary fault, such as a brief power surge, from de-energizing the power line. Only if the overcurrent fault persists does such a protection device de-energize the power line. Some of these arcing faults may initialize the timing circuits of the overcurrent protection devices but, by the end of the time delay, the high impedance nature of the fault limits the fault current to a low value. Conventional overcurrent protection devices cannot distinguish the high variability and low magnitude of the fault current from the levels of current ordinarily drawn by customers; hence, the line may remain energized even though a dangerous fault condition exists on the power line.
Other methods of detecting faults have focused on the high harmonic frequency content of the line current. These earlier methods compared the magnitude values of line current harmonics with a predetermined reference magnitude value. Two randomness techniques are proposed in the following two articles: W. H. Kwon, G. W. Lee, and Y. M. Park, "High Impedance Fault Detection Utilizing Incremental Variance of Normalized Even Order Harmonic Power," IEEE Transactions on Power Delivery, Vol. 6, April 1991, pp. 557-63; and R. D. Christie, H. Zadehgol, and M. M. Habib, "High Impedance Fault Detection in Low Voltage Networks," IEEE Paper No. 92 SM 507-4PWRD, presented at the IEEE-PES Summer Power Meeting, Seattle, July, 1992. The Christie publication mentions that they have implemented a randomness technique, but no specific algorithm or approach is shown.
The Kwon technique implements a calculation based upon the even order harmonics only, using a set of slow-acting calculations and algorithms. Kwon's technique involves calculation of only the difference of the power of even order harmonics in successive cycles, called the "incremental variance." If this incremental variance is sufficiently high for a sufficient number of cycles, a fault is detected. Clearly, Kwon's approach could be ineffective on faults which demonstrate slowly-changing harmonic levels. Kwon's approach is implemented only on even-order harmonics, ignoring all other harmonics, as well as all non-harmonics.
U.S. Pat. No. 3,308,345 to Warrington detects faults having an appreciable harmonic content, including arcing faults. Warrington monitors the magnitude of a distribution circuit's current at frequencies above the third harmonic by first filtering out the fundamental frequency (e.g. 60 Hertz in the United States and 50 Hertz in Europe) and its second and third harmonics. The magnitude values of the remaining high harmonic frequencies, i.e., the fourth, fifth, etc. harmonic frequencies, are then compared to a predetermined threshold magnitude value. Warrington measures the signals over a predetermined length of time and identifies only one frequency range. If the magnitude value of the high harmonic frequency components exceeds a predetermined threshold, and remains above this threshold for a predetermined length of time, then the Warrington device produces a warning signal.
However, faults often exhibit high variability in magnitude at low frequencies, particularly at nonharmonic frequencies near the fundamental frequency. The earlier methods failed to recognized the high variability of these arcing faults from one half-cycle of the fundamental frequency to another, and thus, were ineffective for detecting many arcing faults.
Moreover, arcing faults may become quiescent for brief or lengthy periods, with no measurable fault current being drawn even though the fault condition still persists. The earlier methods ignored this phenomenon.
Also, if the earlier detection systems set the reference magnitude values too low, then they would often be too sensitive. As a result, the power lines would be de-energized when no hazardous fault existed on the line. Conversely, if the reference magnitude values were set too high, the lines would remain energized even though a dangerous fault existed on the power line.
Thus, a need exists for an improved fault detection system for electrical power utilities which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.